Update custom fields

gistfile1.txt

function updateCustomFields()
{
  var data = SpreadsheetApp.openByUrl("YOURDOCURLHERE").getSheets()[0].getDataRange().getValues();
  
  for(var i = 0;i < data.length;i++)
  {
    new Card({link: data[i][0]}).setCustomFieldValue("Unique ID",data[i][1]);
    Utilities.sleep(2000);
  }
}